
struct Town {
    var population = 5_422
    var numberOfstoplights = 4

    func printDescription() {
        print("Population: \(population); number of stoplights: \(numberOfstoplights)")
    }

    mutating func changePopulation(by amount: Int) {
        population += amount
    }
}

var myTown = Town()
myTown.changePopulation(by: 500)
myTown.printDescription()

